Laboratory study on the properties of plastering mortar modified by feather fibers

2.1 Materials

The cement used was of the Chinese Composite Portland Cement type PC32.5 (fineness 80%; equivalent alkali 0.6%, SO₃ 3.5%, and MgO 5.0%, as well as given amounts of C₃S, C₂S, C₃A, C₄AF, and gypsum). The aggregate was mid-sized crushed stone with a maximum diameter of 2.5 mm and a fineness module of 2.3–3.0. The mixing water was pure tap water from the city of Xiangyang, Hubei. The feather fibers used were poultry chicken feathers featuring an average water absorptivity of 30%. The average density of feather barbules (or hooklets) and barbs was 1.326 and 1.459 g/cm³, respectively, as measured by the TD-1200 density tester (degassing method, Builder Electronics Inc., Irvine, CA, USA). The average specific surface area of feather barbules (or hooklets) was 0.479 m²/g (SSA-3500 tester by Builder Electronics Inc.), whereas that of feather barbs was almost three times as high (1.226 m²/g). As the feathers from different parts of poultry vary greatly in diameter and in length, their direct application in mortar is inconvenient. Instead, this work first discarded large rachis fibers and then separated barbules (or hooklets) from barbs. The barbules (or hooklets) were used as received, whereas the barbs were cut to no more than 1 cm in length before they were introduced into the mixture (as shown in Figure 1).

Figure 1

Feather fibers used.

2.2 Specimens

The proportioning of plastering mortars followed the Chinese Technical Specifications for Cement Plastering Mortar [9], with cement content of 400 kg/m³, aggregate content of 1500 kg/m³, and water content of 300 kg/m³. The dosage of feather fibers varied in the range of 0–1.2% by mass of dry mixture (i.e., cement and aggregate). The fibers, cement, and naturally dried aggregate were first mixed homogeneously, then water was added before the mixture was mixed homogeneously. For testing of porosity and compressive strength, mortar cubes were fabricated in sizes of 70.7 mm long by 70.7 mm wide by 70.7 mm height. The mixed fresh mortar was introduced into the mold, then uniformly rodded by a bar from the edges to the center of the mold, and finally surface finished. The mortar was placed under 20±5°C for 24±2 h, de-molded, and then further cured under standard conditions for 28 days±3 h. For testing of thermal conductivity, mortar panels were rodded and cured following the above-described steps, but featured a thickness of 30 mm, a width of 300 mm, and a length of 300 mm. For each measurement described in this study, three duplicate specimens were tested.

2.3 Consistency measurements

The consistency value of each freshly mixed mortar was measured using a consistometer, following the Chinese Standard Test Method on the Basic Properties of Construction Mortars [10]. This was used as an acceptance test.

2.4 Porosity measurements

The actual dimensions of each hardened mortar specimen were accurately measured so as to calculate its volume, V. Then, the mortar was oven-dried and crushed into fine particles, the compacted volume of which (V₀) was then measured using Lee’s density bottle and kerosene method and following the immiscible displacement principle. Finally, the porosity of the mortar (P) was calculated using Eq. (1):

2.5 Compressive testing

The compression test of hardened mortar specimens followed the Chinese Standard Test Method [10]. A YES-300 machine (Jinan Test Machine Co., Shangdong, China) was used for applying the compressive load at 0.5–1.5 kN/s. The compressive strength was calculated by dividing the peak load by the load-bearing surface area.

2.6 Thermal conductivity measurements

The thermal conductivity of hardened mortar specimens was measured using the DRP-6W flat-panel thermal conductivity tester (Xiangke Instruments Co., Hunan, China) shown in Figure 2. This tester is suitable for measuring materials with poor thermal conductivity or materials for thermal insulation. It uses two flat panels with a 33-mm-thick test specimen in between. The temperature at the two flat panels was regulated to be 30°C and 45°C, respectively. Once the temperatures reached these preset values, the thermal conductivity of the mortar (λ) was calculated using Eq. (2):

Figure 2

DRP-6W flat-panel thermal conductivity tester.

where k is the efficiency of the main heater, assumed to be 1.0 in this case; U is the electrical voltage of the main heater (in volts); I is the electrical current of the main heater (in amperes); d is the specimen’s thickness (in meters); t₁ and t₂ are the temperature of the hot and cold plates, respectively (in Kelvin).

3.1 Consistency value of fresh mortars

The consistency value of fresh mortar mixes is shown in Figure 3, which exhibits a strong negative linear relationship between consistency and feather fiber content. The incorporation of feather fibers clearly reduced the consistency value of fresh mortar, and similar results have been reported in wall-protecting mud modified by poultry feather fibers [11]. These results demonstrate the role of feather fibers in inducing plasticity and reducing flowability of fresh mortar. When used as the top surface layer, the plastering mortar should have a consistency value in the range of 70–80 mm to ensure constructability and workability, according to the relevant Chinese Technical Specifications [9]. This corresponds to the fiber dosage of 0.2–0.7% by mass of dry mixture, as shown in Figure 4.

Figure 3

Consistency value of fresh mortar as a function of feather fiber content.

Figure 4

Appearance of mortar panels with various feather fiber contents (scaled down by 1:5).

3.2 Visual appearance of mortar panels

The visual appearance of hardened mortar panels is shown in Figure 4, where panels (A)–(E) indicate the plastering mortar with feather fibers admixed at 0%, 0.2%, 0.4%, 0.8%, and 1.2% by mass of dry mixture, respectively. The general observations are provided as follows. The non-modified mortar panel (A) featured a smooth surface with a small concentration of pinholes and microcracks. As the content of admixed feather fibers increased from 0.2–1.2%, there was a steady increase in the surface roughness of mortar panels. Coincidently, the rougher mortar surfaces exhibited the apparent presence of exposed feathers. This can be explained by the upward migration of lightweight fibers during rodding. It is well known that a rougher surface tends to facilitate diffuse reflection of sound waves and thus reduce their echo and improve their absorption by the surface. A rough surface also facilitates its adhesion with the top plastering layer. A close examination found no presence of cracks on the surface of modified mortar panels, which can be ascribed to the beneficial role of feather fibers in reducing shrinkage and associated cracking [12].

3.3 Porosity of hardened mortars

The porosity of hardened mortar cubes is shown in Figure 5, which exhibits a strong positive linear relationship between porosity and feather fiber content. The incorporation of feather fibers clearly increased the porosity of hardened mortar. This can be attributed to the inherently porous and hollow microstructure of feather barbs and rachis [13]. The feather fibers encapsulate not only air in themselves, but also their interfaces with cement paste or aggregate may induce localized air voids. These mechanisms underlying the porosity changes can also have significant implications in the mechanical properties and thermal-insulation performance of hardened mortars.

Figure 5

Porosity of hardened mortar cubes as a function of feather fiber content.

3.4 Compressive strength of hardened mortars

The compressive strength of hardened mortar cubes is shown in Figure 6, which shows a substantial decrease with the increase in the feather fiber content before leveling off. The non-modified mortar showed a compressive strength of 25 MPa, which is reasonably low in light of its high water-to-cement ratio (w/c=0.75). The incorporation of feather fibers clearly compromised the compressive strength of hardened mortar, and even at a dosage as low as 0.2% by mass of dry mixture it led to a strength reduction of more than 50%. Such reduction induced by the feather fibers is much more severe than that reported previously [7], in which the incorporation of feather fibers at 1% by volume in a concrete with w/c of 0.6 reduced its 28-day compressive strength by 13%. While the difference could be partly attributed to the difference between mortar and concrete, it may also be partly attributed to the different physicochemical properties of the fibers tested.

Figure 6

Compressive strength of hardened mortar cubes as a function of feather fiber content.

Figure 7 shows a typical fracture face of hardened mortar cubes with admixed feathers, after the compression test. Similar to the non-modified mortars, the fracture surface of modified mortars features breaking along roughly 45° directions, which suggests that the incorporation of feather fibers did not significantly change the deformation mechanics of cement mortar.

Figure 7

Typical fracture face of hardened mortar cubes with admixed feathers.

According to the relevant Chinese Technical Specifications [9], the reported level of compressive strength makes the feather fiber-modified cement mortar suitable for two applications as follows. In the absence of adhered decorative bricks, the feather fiber-modified cement mortar can be used for plastering inner wall with the substrate being autoclaved sand-lime brick or aerated concrete block with MU10 strength or for plastering outer wall with the substrate being aerated concrete block. For other applications, we anticipate that future work will focus on methods to greatly improve the strength properties of such mortar mixes. One method would be the mechanical or chemical treatment of feather fibers or modification of cement paste aimed to improve the bonding of fiber/paste interface and to reduce the attack of fiber surfaces by the highly alkaline pore solution in mortar [14].

3.5 Thermal conductivity of hardened mortars

The thermal conductivity of hardened mortar panels is shown in Figure 8, which shows a substantial decrease with the increase in the feather fiber content before leveling off. The non-modified mortar showed a thermal conductivity of 0.4 W/m·K, which is reasonably low in light of its high w/c (0.75). The incorporation of feather fibers clearly enhanced the thermal-insulation ability of hardened mortar, even at a dosage as low as 0.2% by mass of dry mixture. With the feather fiber dosage in the range of 0.2–0.8%, the thermal conductivity of modified mortar remained consistently under 0.25 W/m·K, which is slightly higher than that of thermal-insulation materials made of mostly expanded perlite (0.13 W/m·K) [15]. The thermal conductivity of feather fibers has been reported to be 0.024 W/m·K, close to that of air. The thermal-insulation capacity of both expanded perlite and feather fibers is derived from the low thermal conductivity of encapsulated air. Another mechanism underlying the low thermal conductivity of feather fiber-modified mortar may be related to the air stored at the fiber/paste interface. The fiber/paste interface features inherently poor bonding as well as chemical attack of fiber surfaces by the highly alkaline pore solution in mortar [16].

Figure 8

Thermal conductivity of hardened mortar panels as a function of feather fiber content.

Once the fiber content exceeded 0.2% by mass of dry mixture, its further increase did not show a significant effect on the thermal conductivity of modified mortar (Figure 8). This may be explained as follows. Once the fiber content exceeds a certain level, the microstructure of hardened mortar becomes dominated by interconnected air voids and further increase in porosity no longer significantly affects its thermal conductivity. Furthermore, as the fiber content increased, more fibers showed up in the surface layer of hardened mortar. In other words, the concentration of feather fibers in the inner bulk of mortar remained somewhat stable, due to the light weight of such fibers. These mechanisms might have contributed to the leveling off in the trend of thermal conductivity and compressive strength. As shown in Figure 9, a cross-sectional examination of the mortar cubes confirmed the gradual change in the concentrations of fibers and air voids along the direction of thickness. Typically, the thermal conductivity of thermal-insulation mortars is in the vicinity of 0.29 W/m·K [17]. As such, the feather fiber-modified mortars (with conductivity of <0.25 W/m·K) can be used for the thermal insulation of inner or outer walls.

Figure 9

A cross-sectional examination of the mortar cubes confirmed the presence of more feather fibers on the surface layer.

Future work will explore ways to improve the dispersion of fibers in the mortar or to partially replace the aggregate with porous rock [18] or slag, so as to further reduce the thermal conductivity of modified mortar and potentially improve its mechanical properties as well.